This application relates generally to medical instruments and methods of use for surgery and tissue excision, specifically relating to the removal of cartilaginous and bony tissue. A preferred embodiment more particularly concerns a device useful for preventing tissue damage and cell necrosis during a coring procedure in cartilaginous and bony tissue.
It is well known to utilize a hollow, annular drill bit, such as a trephine cutter, to cut into hard tissue, especially bone, as has been well documented in the prior art. For example, U.S. Pat. Nos. 3,609,056; 4,142,517; 4,782,833; 5,324,300; 5,423,823; 5,697,935; and 6,488,033, all describe annular cutting devices with saw teeth. Frequently, these devices are utilized in combination with either high speed drilling, axially impacting with a mallet, or a combination of both. Devices used in such a manner are not consistent with preserving cell viability in the affected tissue areas, as will be discussed below.
It is also well known to utilize coring devices, such as trephine cutters, to penetrate both soft and hard tissues (e.g., cartilage and bone, respectively) such as may be required in an osteoarticular transfer system (OATS) procedure. Osteoarticular transfer system (OATS) is a surgical method of repairing articular cartilage defects with hyaline cartilage. Limitations of the technique include the inability to deal with large and deep osteochondral defects, limited availability and possible damage to donor sites, non-filled spaces between the circular grafts and incomplete integration of the donor and recipient cartilage. The process for creating a plug or core typically results in an unfavorable level of cell necrosis. The instruments and techniques utilized in the practice of such a procedure may have considerable effect in minimizing the possible damage to the donor sites and thereby encouraging the integration of donor and recipient cartilage.
Historically, prior art devices have focused on the mechanics of cutting into tissues as if they were monolithic inanimate materials. In fact, tissues are living responsive populations of cells that have organized themselves, along with their extracellular matrices (e.g. proteins, glycosaminoglycans, etc.) into complex multiphasic architectural structures. These multiphasic structures have evolved to provide unique biologic and mechanical characteristics to the various tissues (e.g. bone, cartilage, skin, ligament, etc.). In particular, articular cartilage is one of the most micro-architecturally complex tissues in the body, as it features organized structure, instantaneously (i.e. not gradually) transitioning from a tangential orientation to a radial orientation, which then seamlessly flows into a calcified region, forming the osteochondral bond, and finally ending in subchondral cancellous bone. For these reasons, articular cartilage is a difficult tissue to repair. Adding even more complexity is the fact that that the subchondral bone region is an oxygen rich, high cell population, highly vascularized region (e.g., micro-vasculature) that is juxtaposed to an avascular, low cell population, low oxygen zone.
It has long been recognized in orthopedic operations that excess heat derived from drilling results may result in thermal injury and/or necrosis of nearby tissues. It is theorized the injury or necrosis may result from the denaturation of key enzymes required by the bone tissue, thermal damage to the cellular membrane, or the mechanical reorientation of collagen molecules upon exposure to elevated temperatures. Natali et al. describe a study of various bit designs for orthopedic use, measuring temperature increases as the drilling is performed in cortical bone. Elevated temperatures, even for relatively brief periods are capable of causing cell death. Microscopic studies of living bone tissue have shown a high sensitivity to heat stress, for example, Firoozbahsh et al. cited research reporting on the exposure of bone tissue to a temperature of 47 C for one minute, noting the effect of severely impaired bone regeneration.
Drilling designs for bone surgery devices have been incorporated into medical devices, especially trephine cutters. These types of devices using high speed drilling generate excessive heat due to friction between the blade and the material. Furthermore, research has shown that powered trephine devices, when utilized for cartilage grafting, frequently cause gross damage, such as shredding the soft tissue adjacent the bone, creating ragged graft edges, and possibly separating the cartilage from the subchondral bone. These ragged edges are indicative of the damage to which the cartilage tissue has suffered. Damage to this intricately oriented cartilage tissue, whether structural or mechanical, alters the tissue's ability to transmit loads. The creation of these ragged edges, architecturally affects the tissues ability to transmit the forces generated, for example during gait, and therefore increases the localized forces the surrounding cells are exposed to, leading to cell necrosis. Further study of these ragged edges demonstrate decreased cell viability, as the structure of the cells may be damaged.
In addition to the effects of thermal necrosis, and physical trauma (e.g., gross damage to tissue architecture), there is the potential for pressure necrosis when utilizing trephine cutters known in the art. For example, as described in U.S. Pat. Nos. 5,919,196 and 6,592,588, the coring tool or cutting tool is driven into the tissue by a mallet, which, upon impact forces the tool to penetrate the bone. The impacting force generates extreme localized pressure, which can cause pressure necrosis (physical damage to the cell) or injury of the surrounding tissues. Pressure necrosis in bone tissue may occur as cells exposed to a rapid pressure change (e.g., hammer blows, etc.) are injured (e.g. lysed, ruptured, etc.) as a result of the pressure change. The damage may occur in rigid tissue, as bone cells are injured by high forces, though it is believed that the rigid structure of bone would offer some protection to bone cells adjacent to the impacted area. The damage to adjacent non-rigid tissue may be more widespread than that of rigid tissue, as, with a non-rigid tissue (e.g. cartilage) the pressure force may be transmitted further by the non-rigid tissue as the soft tissue is displaced and deformed, rather than shielding the cells.
The prior art (for example U.S. Pat. Nos. 3,577,979; 4,649,918; 5,782,835; 6,007,496; and 6767,354) describes annular cutting edges where one side of the cutting edge (whether an inside wall or outside wall) is vertical, and the other side has been sharpened, machined or manufactured to create a sharp blade. These designs, while they may be used for rigid tissue, suffer from a tendency for the edge to fold over, creating a dull edge. As the edge folds over, dulling the cutting blade, the potential for damage increases as greater friction and gross damage occur to soft tissue or hard tissue to which the tool is applied. Accordingly there is a need for a cutting surface that does not tend to fold over when applied against rigid tissue such as bone, preserving the original sharpness of the blade and therefore minimizing damage to osteochondral cells.
Johanson et al. in U.S. Pat. No. 6,767,354 describe a bone implantation apparatus featuring a harvesting tube and a cutting sheath which are arranged to harvest a bone plug from a donor site, and implant the plug into a prepared implant site. Johanson et al. describe preparing the implant site and the recipient hole for receiving the plug by means of applying a drill bit through a drill guide applied against the surface of the implant site, potentially causing gross damage as the hole is created. Johanson et al. do not describe a cutting tool having serrations to more easily penetrate bony tissue with his device when rotated, as the smooth cutting surface he describes would merely skate over the hard surface when rotated, thereby increasing the chance of damaging adjacent soft tissues. Johanson et al. describe an orthogonally extending tooth, penetrating into the inside bore of the device, which upon being driven, would create a channel in the core plug as the device is driven into the hard tissue. Upon reaching an appropriate depth of penetration, the tip is rotated, such that the tooth causes the plug of material to be severed at the base of the plug.
Spranza in U.S. Patent Application Publication No. 20030199879 describes a bone coring device that is fabricated with a cutting end having a thicker section wall at its distal surface to be placed against tissue, and narrowing towards the proximal end of the tooth, in order to minimize friction between the shank of the device and the adjacent tissue. This thicker cutting end makes the device unsuitable for cutting soft tissues, as the increased width of the cutting edge will impact a large area of soft tissue, and generate a wider swath of gross damage. In order to penetrate harder tissues, a great deal of axial pressure will be required as the force is distributed over a thicker cutting edge than would be the case with a sharper knife-edged coring device. The device described by Spranza requires application of a uni-directional cutting force in order for the tool to be effective.
There exists a need for coring tools useful for articular cartilage wherein the cutting devices are capable of slicing through the tough protective tangential zone, delicately separating the shock absorbing columns of cells in the radial zone and finally cutting into the hard underlying bone. All this must be done in a way that preserves the limited cell population in the cartilage zone; prevents excessive tissue debris that attracts macrophages that could release or stimulate angiogenic factors in the cartilage region causing it to calcify; and still preserves the micro-vasculature of the subchondral bone. To the best of applicants' knowledge, no prior art has combined these aspects into a unique single cutting tool that is sympathetic to both the multiphasic micro-architectural characteristics of the tissue and the biologic requirements of the cell populations which exist in those architectures.
The prior art described does not disclose a device suitable for coring a combination of soft and rigid tissues, while adequately preserving cell viability. It is the intent of this invention to overcome the shortcomings of the prior art in creating a coring tool having features that minimize the occurrence of cell necrosis, preserves cell viability in coring both soft and rigid tissues and in some embodiments is strong enough to allow repeat usage.